Effect of moisture on energy-size reduction of lignite coal in Hardgrove mill

Abstract The effect of moisture on Shengli lignite breakage behavior and energy efficiency was studied experimentally using a standard Hardgrove mill fitted with a wattmeter. The grinding process concerned both the inputs, namely the occurrence and content of water and instantaneous energy consumption and the outputs, size-reduction and product fineness. Results show that the energy-size reduction process for grinding lignite is markedly influenced by moisture occurrence and content. Removing surface moisture from 37.90% to 16.61% (the air-dried condition) resulted in a slight increase of input energy by 0.04 kWh.t−1 per 10 s. However, with further drying inherent moisture to 0%, the consumed energy significantly increased by 0.16 kWh.t−1 per 10 s. Meanwhile, the mass fraction remained on top size decreased from 42.29% to 24.73% and then to 13.00%, while the pulverized coal production increased sharply from 6.28% to 10.68% and to 23.64%, both at a grinding time of 6 min. The energy efficiency was also significantly improved as the moisture content of lignite was reduced to below the air-dried level. The air-dried moisture content was the inflecting point for lignite grinding in the Hardgrove mill. A two-stage pre-drying system was proposed accordingly.